Mousse, Buttercream, And Or Fillings Question?

When watching these Cake shows. I see a thick looking mousse or whipped filling in between the layers. Sometimes its white with fruit, candy pieces, or sometimes Chocolate. I usually have thinner layers of filling, and frostings in mine, because the weight of the fondant makes my cakes bulge. Can someone tell me what they are using please. Thank you.

im worried about that too, last two tier cake I used they requested chocolate mousse filling on the bottom layer so i found a good recipe at recipezaar which was for whipped ganache and it was basically the same thing (next time i'll add more sugar, it was very chocolatey) but it held up fine. I did have to refridgerate it though...HTH